The present invention relates to a method for determining the life of a laser light source for use with an exposure apparatus and a light source thereof to be used in a photolithography process for the manufacture of semiconductor devices, liquid crystal display units, thin film magnetic heads and so on and, more particularly, to a method for determining the life of an excimer laser light source, particularly such as KrF or ArF, etc.
In recent years, as integration of semiconductor devices is improved, there is the tendency that the width of the minimum pattern line of a circuit tends to become narrower. In order to compete with such tendency, excimer laser beams have been employed as a light source for exposure of exposure apparatuseses, in place of mercury lamps which had so far been used primarily for a light source for exposure. The exposure apparatus using excimer laser beams comprises an excimer laser light source and a main body of the exposure apparatus. The excimer laser light source is connected to the main body of the exposure apparatus through an interface cable, such as optical fiber, etc., and designed so as to emit laser beams in accordance with a sequence of control units on the main body side of the exposure apparatus.
Excimer lasers can generate laser beams as pulse light in the order of nanoseconds by the reaction of halogen gases with inert gases by way of discharging in a laser chamber in which a mixture of three kinds of gases are generally filled, including halogen gases such as fluorine gases, etc., inert gases such as krypton or argon gases, etc. and rare gases such as helium or neon gases, etc.
Upon generating laser beams in a repetitive way, the problems may arise that the concentration of the halogen gases may be decreased and consequently a decrease in pulse energy of the laser may be caused due to the connection of the halogen gases to impurities produced in the chamber or adsorption thereof to an inner side of the chamber and further that this may cause a deterioration of each of the structuring parts of the excimer laser light source. Moreover, as ultraviolet light of a high light intensity may also be caused to emit, a window through which to pass the laser beams and a beam splitter, etc. may also cause deteriorating.
Where the excimer laser beams are used as the light source for the semiconductor exposure apparatus, a variation in pulse energy may suffer from inconveniences and difficulties that the accuracy of controlling an mount of exposure onto a photosensitive substrate is reduced, the function of reducing an interference fringe on the photosensitive substrate resulting from an optical system is decreased, or a S/N ratio of signals of a photoelectrical detection system such as a pulse energy monitor system or of an alignment system is decreased, etc. In order to fail to cause such inconveniences and difficulties and to sustain the pulse energy at a constant level, the excimer laser beams undergo feedback to discharging applied voltage, while monitoring the pulse energy that decreases upon a reduction of the concentration of the gases, and the discharging applied voltage is gradually increased. Further, as there is the upper limit of the discharging applied voltage, the gases are exchanged upon reaching the applied voltage to its upper limit in order to return the concentration of the gases to its appropriate value, thereby causing the applied voltage to decrease and sustaining the pulse energy at a constant level.
The deterioration of each of the structuring parts of the excimer laser light source is managed on the basis of the number of emission pulses of the excimer laser beams so that, when the number of the emission pulses thereof has reached a predetermined value, it is decided that the main body of the excimer laser light source or each of the structuring parts thereof is to be exchanged as the life of the main body of the excimer laser light source or each of the structuring parts thereof would have expired. The number of the emission pulses is usually managed on the basis of the counts thereof measured within the excimer laser light source. For example, when the number of emission pulses measured reaches 90% of the pulse number predetermined as an expiration of the life, the excimer laser light source is designed to generate a maintenance request signal of a request for maintenance through an interface to a control unit on the side of the exposure apparatus, thereby giving information that the life of the laser light source or the structuring parts thereof is approaching and expires within short. The pulse number for determining the life of the excimer laser light source or the structuring parts thereof may be predetermined by estimating the life thereof, for example, on the basis of a specification made by a manufacturer of the excimer laser light source or on the basis of the results of tolerance tests based on management standards on the side of a user who uses excimer laser beams as a light source for the exposure apparatus or from the results of accumulated experiences upon using the excimer laser light source, etc.
Generally, however, a laser oscillator of a type in which gases are filled in a laser chamber and the light is activated has a variation in the life with each laser oscillator to some extent. A similar tendency can be seen in excimer laser. As the excimer laser light source uses fluorine gases having a very high reactivity and ability to cause corrosion as well as oscillates ultraviolet light having a high irradiation energy, the life of the structuring parts of the excimer laser light source may therefore vary not only with the frequency of uses of excimer laser beams but also with other various conditions for usage. Thus, the management of the life of the excimer laser light source and the structuring parts thereof only on the basis of the number of the emission pulses can be said to simply understand a rough time of the life with the frequency of uses of the excimer laser light source alone taken into account.
In instances where the life of the excimer laser light source or the structuring parts thereof loaded in the exposure apparatus is managed simply on the basis of the number of the emission pulses in the manner as described hereinabove, there may be caused the occasions that either of the main body of the excimer laser light source or the structuring parts thereof has been determined as deteriorating and as reaching its life by the number of the emission pulses that is actually smaller than the number of the emission pulses predetermined as having reached its life, thereby causing a forcible termination of operating the exposure apparatus at an unexpected time or causing useless exchanges of the excimer laser light source or its structuring parts even if their performance may not yet deteriorate and they can still be used up to the number of the emission pulses exceeding the pulse number determined to have reached the life thereof. These may cause a decrease in throughputs of the exposure apparatus and an increase in costs for manufacturing the exposure apparatuses.